In recent years, lithium ion secondary batteries with high energy density have received great attention for use in small electronic devices, hybrid cars, electric vehicles, and stationary power sources for power storage. Among them, a lithium ion secondary battery using an inorganic solid electrolyte is expected to be a safe battery since there is no concern for liquid leakage of an organic electrolytic solution or gas generation. In addition, as compared to batteries using liquid electrolyte, the lithium battery using the solid electrolyte has a low possibility of side reactions other than battery reactions, and thus, long life span of the battery can be expected. Further, in all solid battery using an inorganic solid electrolyte, it is easy to stack an electrode and an electrolyte to configure the battery, thereby reducing manufacturing cost. At the same time, when the inorganic solid electrolyte is used, a bipolar type battery is able to be configured. As a result, it is possible to expect a higher energy density as compared to conventional batteries using liquid electrolyte. However, in a lithium ion battery with particularly high electromotive force, a positive electrode is a material having a high oxidizing power and a negative electrode is a material having a high reducing power. Therefore, these materials are required to be stable even if the materials and the solid electrolyte are closely adhered. Further, inexpensive constituent elements become an important factor in view of practicality.
In recent years, a lithium lanthanum zirconate based compound capable of being easily obtained by a solid-state reaction in air and having high lithium ion conductivity has received attention. A representative example of this material includes Li7La3Zr2O12. This compound is known to exhibit a high lithium ion conductivity of 5×10−4 S/cm or more which is the top class in an oxide solid electrolyte, at room temperature. In addition, this compound is characterized by excellent electrochemical stability as compared to that of conventional solid electrolytes. On the other hand, since lanthanum, which is a rare earth element, is included in constituent elements of this compound, it is difficult to produce this compound stably and inexpensively, such that practical use becomes a big problem. Therefore, solid electrolytes having a configuration without including expensive elements have been developed, but it is known that the solid electrolytes have conductivity at room temperature of about 4.9×10−5 S/cm, which is about 1/10 lower than that of the lithium lanthanum zirconate based compound. In addition, in the solid electrolyte, conductivity at a low temperature is lower than conductivity at room temperature, which is a problem for practical use.